In the area of thermal printer, it is wellknown that the use of such printers is increasing for certain operations that require a reduction in noise levels and at a reasonable cost. While development work is continually progressing to increase the speed of operation with minimum maintenance or care of the equipment, it is seen that improvements are being made to provide a higher quality print with a reduced number of thermal elements.
In the case of thermal printers, it is necessary to minimize the complexity of the thermal print head and the associated electronic controls. In certain prior are printers, it has been common practice to provide a thermal print head having a plurality of thermal printing elements on the surface of the head and wherein selected elements are energized to provide printing on thermal paper of like record media with the print head operating in a stationary or fixed position relative to the printer frame. The thermal printing elements on the print head may take the form of pads or contact surfaces in the shape of characters and connected by conducting runs to side or edge connectors. A flexible flat ribbon-like cable is normally used to connect with the pads or contact surfaces on the print head and the individual leads or wires of the cable may include end connectors or terminals for contact with the print head pads.
In more recent developments, a thermal printer may include and utilize a moving or shuttling print head and while printing speeds are less than thermal printers having fixed print heads, there are certain features or advantages of the shuttling type heads. The moving print head is usually less complex and less expensive than the fixed head. A printer using the moving print head may include a print head having a vertical row of thermal elements in an arrangement for producing characters of seven or more dots in height. Printing speeds of 30 to 50 characters a second are realized in printing one line at a time as the head moves in the direction from left to right along the line of printing. Printing speed is influenced or governed, at least in part, by the amount of time required to pulse or fire and then to cool each of the thermal elements. This amount of time is typically in the range of four to six milliseconds per dot position. A moving or shuttling print head may also be formed wherein the thermal print elements are arranged in a line of dots.
In the case of the fixed print head, the arrangement may also be in a line of dots (the print elements are positioned horizontally on the print head) and by reason, at least in part, due to the greater number of print elements used and also because the print head does not move, printing speeds of 200 or more characters per second may be achieved. Print heads of the fixed type are inherently more costly than the shuttling type and generally are used where higher resolution and higher printing speeds are required.
It should be appreciated that a typical problem in thermal type printers is the accumulation and storage of heat in the higher speed printing. The heat that is generated in the thermal printing elements during induction of current is partially used for the printing process and partially radiated through the substrate of the print head. At printing speeds generated in the range wherein the pulsing or firing cycle of operations is below 10 milliseconds, such as the four to six milliseconds per dot position mentioned above, the next printing pulse may be initiated before heat has been sufficiently radiated or dissipated from the thermal printing elements and/or substrate.
It is thus seen that heat is stored or is still retained in at least some of the heater or print elements resulting in non-uniform and uneven temperatures among the respective elements in the printing or recording operation so that the printed dots may be different from each other in size and/or density. The printing density may be affected by the number of print elements which are pulsed or energized at the same time.
It is also known that when multiple print elements are located and/or positioned on the substrate, and are fired in sequential manner to effect printing, the average temperature of the print head substrate increases as a function of time until sufficient residual heat in the substrate causes degradation of the print quality, resulting in such difference in size and/or density of the dots.
The problem of such degradation of the print quality from the increase of average temperature generated from the thermal head for printing cannot be prevented by uniform control of the entire print head nor can such problem be prevented with respect to individual print elements. The problem of increasing temperature due to the use of the thermal printing apparatus over a prolonged period of time can also result in damage to the thermal print head.
Representative documentation in the area of thermal printing and of problems associated with increasing print head temperatures includes U.S. Pat. No. 4,462,704, issued to M. Kurata et al. on July 31, 1984, which discloses a thermal head driving system having a plurality of thermal heads and wherein a pulse generator is provided between an electric power source and each of the heads for producing a driving pulse having a width adjustable and determined in accordance with a ratio of the resistance value of each thermal head.
U.S. Pat. No. 4,409,600, issued to M. Minowa on Oct. 11, 1983, discloses a thermal printer drive circuit that includes a current application time control circuit for varying the period of time during which current flows to the heat generating elements. The control circuit includes a charging circuit having a capacitor and a discharging circuit connected in parallel.
U.S. Pat. No. 4,510,507, issued to Y. Ishikawa on Apr. 9, 1985, discloses thermal recording apparatus which provides image quality through control of recording pulses by means of control of pulse width or voltage according to the level of color of the image, and also by control of pulse width at the start or at the end of each recording operation.
U.S. Pat. No. 4,524,368, issued to T. Inui et al. on June 18, 1985, discloses a thermal head drive circuit wherein improved quality is effected by using data from previously printed lines to compute a corrected pulse energy. The circuit uses a heat storage state operator for each heater element, a pulse energy operator for computing a printing pulse energy to be applied to each heater element, a memory for storing the pulse energy used in the previously printed line, and a counter to count the number of dots on the line to be printed.
U.S. Pat. No. 4,573,058, issued to R. M. Brooks on Feb. 25, 1986, discloses a thermal printer for maintaining constant printing energy by sensing voltage proportional to element resistance and developing an average printhead resistance. In response to a change in average printhead resistance, a processor maintains constant printing energy by changing the printing pulse width.
U.S. application Ser. No. 859,515, filed May 5, 1986, invented by R. M. Brooks et al., and assigned to the same assignee as the present invention, discloses thermal printing apparatus having means for measuring the resistance of each individual thermal element, means for selection of individual thermal elements to be energized, and processing means for computing individual thermal element burn values.